Stimulation of glycogen synthesis in hepatocytes by added amino acids is related to the total intracellular content of amino acids

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Abstract
Katz et al. [Katz, J., Golden, S. and Wals, P. A. (1976) Proc. Natl. Acad. Sci. USA 73, 3433-3437] were the first to report that in hepatocytes isolated from fasted rats and incubated with either dihydroxyacetone, glucose or other sugars, glycogen synthesis was greatly accelerated by addition of amino acids. We have looked for possible mediators responsible for this effect and have tested the effect of alanine, proline, asparagine, glutamine or a combination of ammonia with either pyruvate or lactate in activating glycogen synthesis from dihydroxyacetone. The following observations were made. 1. Stimulation of glycogen synthesis by alanine, proline or asparagine does not require production of glutamine since the effect also occurs in periportal hepatocytes which lack glutamine synthetase. 2. Under various conditions, stimulation of glycogen synthesis by added amino acids directly correlated with increases in the intracellular content of amino acids, expressed in osmotic equivalents. 3. 3-Mercaptopicolinic acid, the inhibitor of phosphoenolpyruvate carboxykinase, further enhances stimulation of glycogen synthesis by amino acids because it increases the intracellular accumulation of aspartate and glutamate. 4. The previously reported enhancement by leucine of the stimulation of glycogen synthesis by glutamine [Chen, K. S. and Lardy, H. A. (1985) J. Biol. Chem. 260, 14683-14688] can be ascribed to inhibition of urea synthesis by leucine which results in accumulation of glutamate and of ammonia, the essential activator of glutaminase. It is concluded that activation of glycogen synthesis by added amino acids is due to an increase in intracellular osmolarity following their uptake and the accumulation of intracellular catabolites. This results in an increase in hepatic volume which stimulates glycogen synthesis [Baquet, A., Hue, L., Meijer, A. J. van Woerkom, G. M. and Plomp, P. J. A. M. (1990) J. Biol. Chem. 265, 955-959].